Almost
everyone today has flown in an airplane. Many ask the simple question
"what makes an airplane fly?" The answer one frequently gets is
misleading and often just plain wrong. As an example, most descriptions
of the physics of lift fixate on the shape of the wing (i.e. airfoil)
as the key factor in understanding lift. The wings in these
descriptions have a bulge on the top so that the air must travel
farther over the top than under the wing. Yet we all know that wings
fly quite well upside down where the shape of the wing is inverted. To
cover for this paradox we sometimes see adescription
for inverted flight that is different than for normal flight. In
reality the shape of the wing has little to do with how lift is
generated and everything to do with efficiency in cruise and stall
characteristics. Any description that relies on the shape of the wing
is wrong.

Let
us look at two examples of successful wings that clearly violate the
descriptions that rely on the shape of the wing. The first example is a
very old design. Figure 1 shows a photograph of the Curtis 1911 model D
type IV pusher. Clearly the air travels the same distance over the top
and the bottom of the wing. Yet this airplane flew and was the second
airplane purchased by the US Army in 1911.

Figure 1. Curtis 1911 model D type IV
pusher

The
second example of a wing that violates the idea that lift is dependent
on the shape of the wing is of a very modern wing. Figure 2 shows the
profile of the Whitcomb Supercritical Airfoil (NASA/Langley
SC(2)-0714). This wing is basically flat on top with the curvature on
the bottom. Though its shape may seem contrary to the popular view of
the shape of wings, this airfoil is the foundation of the wings of
modern airliners.

Figure 2. Whitcomb Supercritical Airfoil

The emphasis on the wing shape in many
explanations of lift is based on the Principle of Equal
Transit Times.
This assertion mistakenly states the air going around a wing must take
the same length of time, whether going over or under, to get to the
trailing edge. The argument goes that since the air goes farther over
the top of the wing it has to go faster, and with Bernoulli’s principle
we have lift. Knowing that equal transit times is not defendable the
statement is often softened to say that since the air going over the
top must go farther it must to faster. But, this is again just a
variation on the idea of equal transit times. In reality, equal transit
times holds only for a wing without lift. Figure 3 shows a simulation
of the airflow around a wing with lift.

Figure 3. Air over a wing with lift

The
Bernoulli equation is a statement of the conservation of energy. It is
correct, but not applicable to the description of lift on a real wing.
The wings of an 800,000 pound airplane are doing a great deal of work
to keep the airplane in the air. They are adding a large amount of
energy to the air. One of the requirements of the application of the
Bernoulli principle is that no energy is added to the system. Thus, the
speed and pressure of the air above a real wing in flight are not
related by the Bernoulli principle. Also, descriptions of lift that
evoke the Bernoulli principle depend on the shape of the wing. As
already stated, the shape of the wing affects the efficiency and stall
characteristics of the wing but not the lift. That is left to the angle
of attack and speed.